Some 30% of all proteins known from genomic sequencing are predicted to be found in a cellular membrane. But in contrast to the many structures of soluble proteins that are known, only a handful of membrane protein structures have been determined. We are working to crystallize and determine a structure by X-ray crystallography of a membrane protein. We are trying to produce and to crystallize sufficient amounts of the transporter associated with antigen processing (TAP). This protein actively transports small peptides from the cell's cytoplasm through the endoplasmic reticulum membrane where the peptide becomes bound to an antigen-presenting protein. T cells can respond to the peptide when it is on the antigen-presenting protein. Our approach is to produce the protein in bacteria as the denatured polypeptide chains that bacteria often can produce and then to renature the proteins to their native and functional conformations. After obtaining enough of the pure protein, we will crystallize the protein in preparation for the X-ray crystallographic structure determination. We have begun to express just the parts of the TAP protein that bind ATP and couple energy to the peptide-pumping part of the protein. This will allow us to understand the "motor" part of the transporter protein.